The present invention relates to an image forming apparatus, such as an electrophotographic copier or a printer, and paper feed members thereof. More particularly, the present invention relates to an image forming apparatus and its paper feed members arranged such that a transfer member transfers a toner image formed on an image carrier to an image record sheet.
In an image forming apparatus, a transfer roller, a transfer pad, and a transfer belt, all of which are pressed against the surface of an image carrier, are known as the transfer member. This type of image forming apparatus is provided with a paper feed device for taking sheets for image forming purposes out of a paper feed tray, and carries the thus-taken sheets one by one to a transfer member provided in a downstream direction. In the paper feed unit, there are commonly used a pickup paper member (a pickup roller) for taking sheets for image forming purposes out of a paper feed tray, paper feed members (e.g., a feed roller, a feed belt, or the like) for conveying the sheets taken out by the pickup paper member one by one by separating the sheets from one another in a frictional way, and paper feed members for supporting purposes (a retard roller, a belt, or a pad) which come into contact with the paper feed member for conveying purposes.
For example, a so-called low-polarity rubber material, such as an ethylene-propylene-diene (EPDM) rubber, a silicon rubber, a natural rubber, or an isoprene rubber, has been used as the rubber material that forms the paper feed members such as the pickup roller, the feed roller, or the retard roller. Further, a urethane rubber or an EPDM has been used as the rubber material that forms the transfer roller. Currently, because of its stability with respect to variations in the ambient, a urethane rubber is chiefly used as the rubber material of the transfer roller.
In the image forming apparatus having the transfer roller made of a urethane rubber, imperfections will often arise in the case where the paper feed members (i.e., the pickup roller, the feed roller, or the retard roller) are made of the low-polarity rubber material.
Through studies and research on the reason why imperfections are liable to arise in the image, inventors of the present invention have elucidated the cause and mechanism of occurrence of imperfections in an image.
More specifically, it has been determined that contaminants which become attached to the sheet after having been scraped away from the surface of the paper feed member become attached to the transfer member in a frictional way, and the contaminants are further transferred to the surface of the image carrier, thereby resulting in imperfections in an image such as fog. It has been found that the imperfections in an image greatly depend on the polarity of the contaminants.
According to the studies performed by the inventors, it has been determined that the polarity of the contaminants which induce imperfections in an image does not really depend on the polarity of a compounding agent, such as a softening agent, included in the rubber material of the paper feed member but depend on the polarity of oligomer inherently originated from the rubber material before it is crosslinked.
With reference to FIGS. 13A, 13B, and 14A to 14C, the mechanism of occurrence of imperfections (or fog) in an image will be described.
In FIGS. 13A and 13B, image record sheets 01 taken out of a paper feed tray by a pickup roller are separated from each other one by one in a frictional way by means of a feed roller 02a (for conveying purposes) and a retard roller 02b (for supporting purposes) which are in pressed contact with each other. The thus-separated sheet is conveyed to a transfer area Q3 (formed in the area where a transfer roller 03 comes into pressed contact with an image carrier 04). In this event, a trace amount of contaminants represented by O become attached to the sheet 01 from the paper feed members 02a and 02b.
At this time, in order for the retard roller 02b to apply a brake to the sheet 01, the difference between the sheet 01 and the outer peripheral surface of the feed roller 02a becomes smaller than the difference in speed between the sheet 01 and the outer peripheral surface of the retard roller 02b. More specifically, there is a small amount of slippage between the feed roller 02a and the sheet 01, whilst there is a large amount of slippage between the retard roller 02b and the sheet 01. For this reason, the amount of contaminants that are removed from the surface of the feed roller 02a and then become attached to the sheet 01 is smaller than that of the contaminants that are removed from the surface of the retard roller 02b and then become attached to the sheet 01. FIG. 13A is an illustration in which emphasis is placed on the amount of the contaminants attached to the sheet 01 from the paper feed members 02a and 02b.
In FIG. 13B, the travel speed of the sheet 01 in a transfer area Q3 is usually set to be substantially the same as the peripheral speed of the image carrier (a photosensitive element) 04. The peripheral speed of the transfer roller 03 is usually set to be faster than the travel speed of the sheet 01 by several percentages. For this reason, there is no slippage between the image carrier 04 and the sheet 01, whereas there is slippage between the transfer roller 03 and the sheet 01.
Consequently, when the sheet 01, having the contaminants attached thereto, is transferred to the transfer area Q3, the contaminants are prevented from moving to the surface to the image carrier 04. However, since the transfer roller 03 comes into contact with the surface of the sheet to be brought into contact with the retard roller 02b, the contaminants attached to the sheet 01 are transferred to the transfer roller 03 in a frictional way.
The urethane rubber possesses a high polarity, and paper or OHP paper which is commonly used as the sheet and comprises cellulose as a major constituent, also has a high polarity. Therefore, both the conventional transfer roller 03 made of urethane rubber and the commonly-used sheet 01 have a high polarity.
In contrast, the conventional retard roller 02b is formed from a low-polarity rubber material such as an EPDM. As previously described, the contaminants attached to the surface of the sheet to be brought into contact with the transfer roller 03 also have a low polarity. In general, it is known that there is high compatibility between the substances possessing an identical polarity, whereas there is low compatibility between the substances possessing different polarities.
In FIG. 14A, the transfer roller 03 comes into direct contact with the image carrier 04 before the next sheet 01 arrives at the transfer roller 03 after the sheet 01 has passed through the transfer roller 03 (i.e., during an inter-image period). At this time, there is a difference in peripheral speed between the transfer roller 03 and the image carrier 04. Further, the urethane rubber of the transfer roller 03 is different in polarity from the contaminants attached to the transfer roller 03. Therefore, the contaminants attached to the transfer roller 03 partially move to the image carrier 04. These contaminants gradually accumulate on the surface of the image carrier 04 without being completely cleaned through operation of the image forming apparatus carried out over a long period of time.
In FIG. 14B, during the course of travel of the contaminants attached to the surface of the image carrier 04 through a development area, toner represented by .oval-solid. becomes attached to the contaminants provided on the image carrier 04. In FIG. 14C, the toner attached to the surface of the contaminants provided on the image carrier 04 is transferred to the next conveyed sheet 01, thereby resulting in background fog.